The present invention relates to a novel process for the dehydrocyclodimerization of C.sub.2 to C.sub.5 aliphatic hydrocarbons. Specifically, a process for producing aromatics via the dehydrocyclodimerization reaction, which utilizes a novel start-up method is disclosed.
Dehydrocyclodimerization is a reaction where reactants comprising paraffins and olefins, containing from 2 to 5 carbon atoms per molecule, are reacted over a catalyst to produce primarily aromatics with H.sub.2 and light ends as by-products. This process is quite different from the more conventional reforming or dehydrocyclization process where C.sub.6 and higher carbon number reactants, primarily paraffins and naphthenes, are converted to aromatics. These aromatics contain the same or less number of carbon atoms per molecule as the reactants from which they were formed, indicating the absence of reactant dimerization reactions. In contrast, the dehydrocyclodimerization reaction results in an aromatic product that always contains more carbon atoms per molecule than the C.sub.2 to C.sub.5 reactants, thus indicating that the dimerization reaction is a primary step in the dehydrocyclodimerization process. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500.degree. F. using dual functional catalysts containing acidic and dehydrogenation components. These catalysts include acidic amorphous aluminas which contain metal promoters. Recently crystalline aluminosilicates have been successfully employed as catalyst components for the dehydrocyclodimerization reaction.
An important aspect of any catalytic process is the activity and stability of a catalyst composition when exposed to normal process conditions. The optimization of hydrocarbon process catalyst activity and stability are continuing goals of process and catalyst development efforts.
It has now been found that a dehydrocyclodimerization process utilizing a catalyst comprising a Group IIB-IVB metal component and a crystalline aluminosilicate zeolite exhibits improved initial conversion and aromatic selectivity in a dehydrocyclodimerization reaction if the process is started-up in the relative absence of hydrogen gas.